A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. In many applications requiring a stable and well-regulated output, switched-mode power converters are frequently employed to advantage. Switched-mode power converters generally include an inverter, a transformer having a primary winding coupled to the inverter, a rectifier coupled to a secondary winding of the transformer, an output filter and a controller. The inverter generally includes a power switch, such as a field-effect transistor (FET), that converts an input voltage to a switched voltage that is applied across the transformer. The transformer then transforms the voltage to another value and the rectifier generates a desired voltage at the output of the converter. The output filter, typically an output inductor and an output capacitor, smooths and filters the output voltage for delivery to a load.
There are two common methods of regulating the output voltage of the converter, namely, voltage-mode control and current-mode control. In voltage-mode control, the controller typically includes an error amplifier coupled to the output of the power converter. The controller further includes a modulator coupled between the error amplifier and the power switch. The error amplifier monitors the output voltage of the power converter and generates an error signal representing a deviation between the actual output voltage and a desired output voltage. The modulator then generates a drive signal for the power switch based on the error signal. For example, the drive signal may maintain the power switch in a conduction mode while the error signal exceeds a periodic ramp signal. The drive signal may then transition the power switch to a non-conduction mode when the periodic ramp signal reaches the error signal.
In current-mode control, a current in the power converter, such as a switch current through the power switch or an inductor current through the output inductor, is substituted for, or added to, the periodic ramp signal. The output voltage of the converter is still fed back through the error amplifier circuit to provide a component of the error signal for the modulator. The aforementioned methods and variations thereof are widely used and are adequate for many loads.
Low voltage digital loads that generate fast, large amplitude step changes in output current, however, may cause the error signal to vary considerably within a switching cycle. In conventional voltage or current mode control, the switching cycle may be divided into a primary interval D (during which the power switch is in the conduction mode) and an auxiliary interval 1-D (during which the power switch is in the non-conduction mode). In trailing edge modulation, for example, the primary interval D begins with a timing circuit resetting the ramp signal to the modulator, causing the modulator to place the power switch in the conduction mode. During the primary interval D, the ramp signal continues to rise at substantially constant slope. Then, once the ramp signal reaches the error signal, the power switch is placed in the non-conduction mode to begin the auxiliary interval 1-D. During the auxiliary interval 1-D, the modulator simply waits for the timing circuit to reset the ramp signal to begin a new switching cycle.
The modulator thus exhibits an active decision process during the primary interval D. Any change in the error signal either continues to keep the power switch in the conduction mode (because the error signal is greater than the ramp signal) or places the power switch in the non-conduction mode thereby ending the primary interval D (because the error signal has dropped below the ramp signal). During the auxiliary interval 1-D, however, the modulator must wait for the timing circuit to begin the new switching cycle. Any change in the error signal resulting from changes in the operating conditions of the power converter is effectively ignored, thereby limiting the response of the power converter.
Accordingly, what is needed in the art is a circuit that allows the power converter to more rapidly respond to changes in the operating conditions of the power converter during both the primary and auxiliary intervals of a switching cycle.